Russia has bloodlessly seized control of Crimea over the past week and has moved to formally annex it. The Crimean parliament has scheduled a vote on whether to join Russia for next week.

"I think it's part of a long-term strategy on Putin's part to create a Russian sphere of influence, a Russian bloc," Gates said, adding later: "I don't think he will stop in Ukraine until there is essentially a pro-Russian government in Ukraine, in Kiev."

Gates said he supported efforts by Congress and the administration to increase sanctions on Russia.